Carbon Burial Research Articles

Large Igneous Province Emplacement Triggered an Oxygenation Event at ∼1.4 Ga: Evidence From Mercury and Paleo‐Productivity Proxies

AbstractThe mid‐Proterozoic (∼1.8–0.8 Ga) ocean‐atmosphere system is hypothesized to have experienced fluctuations in redox conditions with transient oxygenation events. One of these happened at ∼1.4 Ga, and it is speculated that this event may link to the emplacement of large igneous province (LIP) at this time. However, direct evidence for this relationship remains to be proved. Here, we report Hg/TOC, P, and trace element concentrations across the ∼1.4 Ga oxygenation event in the Xiamaling Formation of North China. A prominent increase in Hg/TOC is slightly earlier than that of nutrient contents (especially P), pyrite and TOC abundances, suggesting that this distinct oxygenation event was likely the result of LIP activity at ∼1.4 Ga, which increased nutrient and sulfate supply from continental weathering to the ocean, sustaining elevated primary productivity, organic carbon and pyrite burial. This study indicates that LIP weathering could trigger transient oxygenation events during the mid‐Proterozoic.

Blue carbon in sediment from Sanggou Bay: composition, burial flux and its response to human activities

Marine primary production and terrestrial input are the main sources of buried carbon in sediments of marginal seas. Only marine-source carbon buried in sediments, fixed and stored by marine ecosystems, belongs to “blue carbon” and reflects marine ecosystems’ carbon sink function. The pattern of buried blue carbon in sediments, its flux, and its relationship with environmental changes remain unclear. The study aimed to investigate the composition of blue carbon in the sediments of Sanggou Bay, a special type of marginal sea. The analysis of sediment carbon sources was conducted through the C/N ratio and microscopic examination. The study also examined the long-term changes in the blue carbon burial fluxes. Results showed Blue carbon, which is sea-sourced carbon, accounted for about 23% of the total carbon content and its concentration ranged from 0.17% to 0.51%, with an average of about 0.25% ± 0.10%. The content of organic blue carbon in this sea area ranges from 0.09% to 0.26%, with an average of around 0.18% ± 0.04%. It constitutes approximately 72% of the buried blue carbon in the sediment, making it the primary component of buried blue carbon. Meanwhile, the content of inorganic blue carbon ranges from 0.01% to 0.32%. Over the past 70 years, the burial fluxes of sedimentary blue carbon, organic blue carbon and inorganic blue carbon in the Sanggou Bay are about 0.54 ± 0.22 mmol/(cm2a), 0.38 ± 0.07 mmol/(cm2a) and 0.17 ± 0.22 mmol/(cm2a), respectively; their long-term changes have been significantly affected by human aquaculture activities. Large-scale raft-rack aquaculture activities have caused a reduction in water flow velocity and an increase in the deposition of particulate organic matter, which in turn has led to the burial of organic blue carbon in the sediment. Additionally, the competition between aquaculture products and small calcareous organisms, such as mussels, foraminifera, may have inhibited the growth of small calcareous organisms. We suggest this has resulted in reduced burial fluxes of inorganic blue carbon and a decrease in its proportion among total blue carbon in the sea area. Our findings imply that aquaculture activities in Sanggou Bay had a negative impact on the burial of blue carbon in the sediments.

Assessing biogeochemical controls on porewater dissolved inorganic carbon cycling in the gas hydrate-bearing sediments of the Makran accretionary wedge, Northeastern Arabian Sea off Pakistan

Quantitatively assessing the porewater dissolved inorganic carbon (DIC) cycling in methane-enriched marine sediments is crucial to understanding the contributions of different carbon sources to the global marine carbon pool. In this study, Makran accretionary wedge was divided into Zone 1 (high methane flux area) and Zone 2 (background area). Porewater geochemical compositions (Cl–, SO42–, NH4+, Mg2+, Ca2+, Ba2+, DIC and δ13C-DIC) and a reaction-transport model were used to determine the DIC source and calculate the DIC flux through carbonate precipitation and releasing into overlying seawater in sediments. Zone 1 is characterized by the shallower depth of sulfate-methane transition (SMT), where most of porewater sulfate was consumed by anaerobic oxidation of methane (AOM). In contrast, a relatively low flux of methane diffusion in Zone 2 results in a deeper SMT depth and shallow sulfate is predominantly consumed by organoclastic sulfate reduction (OSR). Based on the porewater geochemical profiles and δ13C mass balance, the proportions of porewater DIC originating from methane were calculated as 51% in Zone 1 and nearly 0% in Zone 2. An increase of porewater DIC concentration leads to authigenic carbonate precipitation. Solid total inorganic carbon (TIC), X-ray diffractometry (XRD) and scanning electron microscopy (SEM) analysis display that carbonate content increases with depth and aragonite appears at or below the depths of SMT. Meanwhile, the flux of DIC released from sediments calculated by the reaction-transport model is 51.3 ~ 90.4 mmol/m2·yr in Zone 1, which is significantly higher than that in Zone 2 (22.4 mmol/m2·yr). This study demonstrates that AOM serves as the dominant biogeochemical process regulating the porewater DIC cycle, which has an important impact on the authigenic carbonate burial and the seawater carbonate chemistry.

Palaeoenvironmental changes in the Late Triassic lacustrine facies of the Ordos Basin of Northwest China were driven by multistage volcanic activity: Implications for the understanding the Carnian Pluvial Event

The Carnian Pluvial Event (CPE) induced an intense carbon-cycle disturbance and biological crisis at a global scale. The eruption of the Wrangellia Large Igneous Province (W-LIP) is considered a possible triggering mechanism; however, this cannot fully explain the significant differences in environmental perturbations across different regions. In this paper, we present detailed records of petrography, mercury geochemistry, carbon and sulfur isotopes, trace elements, and provenance data for the Triassic lacustrine facies of the Ordos Basin in the North China Plate, which provide insights into the important effects of volcanic activities, other than the W-LIP, on palaeoenvironment during the CPE interval. Massive emissions of SO2, CO2, and pyroclastic deposits resulting from these volcanic activities triggered four humid episodes, each characterized by lake-level rise, negative δ13Corg excursion, a transition from mudstones to organic-rich shales facies, as well as algal bloom within a time span of 180 kyrs. The establishment of an age framework in the Ordos Basin enables comparative analyses of environmental evolution during the Carnian age across the North China Plate, the western Tethys domain, and the South China Plate. During the period of global climate perturbation driven by the W-LIP eruption, we conclude that short-term localized volcanic activities may have further induced high frequentcy environmental perturbations, which influenced biological shifts and increase organic carbon burial. These short-term volcanic activities with different occurrence times and frequencies complicate local-scale palaeoenvironmental evolutions, which may be an important reason contributing to significant differences in environmental perturbations during the CPE interval across different regions of the globe.

Cross‐Shelf Carbon Transport in the East China Sea and Its Future Trend Under Global Warming

AbstractThe cross‐shelf carbon transports across a section along the 100 m isobath from Taiwan to Cheju are estimated based on carbon concentration observations and ocean currents simulated by a regionally zoomed ocean‐atmosphere coupled model. Results show that the net cross‐shelf particulate organic carbon, dissolved organic carbon, and dissolved inorganic carbon transports are in the offshore direction at 1.81 ± 0.22, 51.8 ± 2.85, and 783 ± 58.9 TgC yr−1, respectively, which are high in spring and summer and low in winter and fall following the seasonality of the offshore volume transport. The carbon is transported into the East China Sea (ECS) mainly by the Taiwan Warm Current and the Yellow Sea Warm Current, whereas they are carried out of the ECS mainly by the East China Sea Current extension and the recirculation north of Taiwan. Assuming steady biological activity, future net total organic carbon transports are projected to increase by 5%∼19% offshore at the end of the 21st century, with the maximum increase in winter, in a high greenhouse gas emission scenario of RCP8.5. The increased carbon transports are due to the increased offshore volume transport, transferring more carbon‐rich coastal water away from the shelf in the warming future than at present. The results quantify cross‐shelf carbon burial in the ECS in the background of global warming.

Relationships Between Land Use and Terrestrial Organic Matter Transfer to the Baltic Sea Over the Last 500 Years

AbstractTerrestrial organic matter (OM) plays a key role in coastal organic carbon burial. However, few studies focus on the relationship between land use in the watershed and the transport of terrestrial OM to coasts from a long‐term perspective. In this study, we compared terrestrial OM deposition between an inlet of the Baltic Sea and an upstream lake within the same watershed over the last 500 years, using lignin biomarkers in the sediments. In combination with pollen‐based quantitative land cover reconstruction, we assessed the impacts of semicentennial‐scale changes in land use on terrestrial OM export. The results indicated that the concentration, composition, and degradation state of the lignin‐derived OM differed substantially between the two sites. The lake received larger amounts of lignin‐derived OM during periods of intensified agriculture, but the coastal site did not. The composition of lignin in the coastal sediment did not directly reflect variations in vegetation cover in the watershed. The reason could be that the OM was settled in the upstream basins. Furthermore, the terrestrial OM that did reach the coastal sediments was modified through degradation during the transport, and only the refractory component was deposited at the coast in a relatively unaltered form.

Ubiquitous occurrence of organogenic dolomite in a late Ediacaran limestone-dominated succession from the Eastern Yangtze Gorges area of South China

The widespread occurrence of dolostones in Paleozoic and Precambrian sequences stands in stark contrast to the difficulty of precipitating dolomite in experiments at low temperature under abiotic conditions. The discoveries of dolomite within modern microbial mats and the successful synthesis of proto-dolomite under ambient conditions catalyzed by various microorganisms and organic molecules lend support to the development of organogenic dolomite models. Despite this, convincing evidence for ancient primary dolomite precipitation is rare, mainly due to post-depositional modifications of original textures and mineralogy. Here we present petrographic, mineralogical and C-isotopic data of carbonates preserving primary sedimentary textures in the late Ediacaran Shibantan Member of the Dengying Formation from the eastern Yangtze Gorges area, South China. These carbonates are characterized by calcite layers intercalated with millimeter-thick, organic-rich layers that contain up to 50 % dolomite. Petrographic observations and Raman imaging show that the dolomite occurs preferentially in microbial mats and is closely associated with organic matter. Scanning electron microscopy also revealed the presence of Mg-rich nanoscopic carbonate aggregates and ribbon-like microstructures akin to mineralized extracellular polymeric substances (EPS) in mat layers. These observations are inconsistent with replacive origins but instead support an organogenic origin for the Shibantan dolomite. The relatively high TOC (total organic carbon) contents and consistently positive δ13Ccarb values (up to +4.8 ‰) suggest enhanced burial of organic carbon produced in microbial mats during the Shibantan deposition. Subsequent organic degradation by microorganisms in anoxic, early diagenetic environments likely promoted the ubiquitous formation of organogenic dolomite in Shibantan microbial mats. Our study highlights the important roles of organogenic dolomites in interpreting the wide occurrence of Paleozoic and Precambrian dolostones.

Recycling and deposition of inorganic carbon from calcium carbonate encrustations of charophytes

AbstractMany aquatic primary producers can use bicarbonates as a carbon source for photosynthesis. Charophytes of the two genera: Chara and Nitellopsis are quite efficient in this process. Some species of these macroalgae produce carbonate encrustations, mainly calcium carbonate, constituting up to 86% of the summer maximum dry weight of the standing crop. In this study, we analyzed the fate of inorganic carbon accumulated this way in Chara spp. and Nitellopsis obtusa from six Polish lakes located in two regions (warmer W Poland and cooler NE Poland). Our study distinguished two groups of charophyte species that differed in the way of CaCO3 release from their summer standing crops. On average, the corticate Chara rudis and C. tomentosa belonging to the first group were less efficient in depositing CaCO3 from summer to autumn than the less corticate C. contraria and ecorticate N. obtusa of the second group. The latter two species were more efficient in inorganic carbon burial in sediments. On the contrary, dissolution of encrustation was more typical of the first species group and was facilitated by decreasing the pH and saturation index of calcite in lake water. The final output of CaCO3 loss mainly resulted from combined species‐specific features, lake water properties and overwintering patterns. Our study revealed that inorganic carbon cycling through charophytes involves burial and dissolution and is more complex than previously thought.

Intrashelf basin record of redox and productivity changes along the Arabian margin of Neo-Tethys during Oceanic Anoxic Event 1a

The biotic, environmental, climatic, oceanic, and sea-level perturbations during the Early Aptian Oceanic Anoxic Event (OAE) 1a have been extensively documented from both deep- and shallow-marine deposits worldwide. However, there has been relatively little comparative assessment of the simultaneous interplay among organic carbon burial, redox conditions, terrigenous output, and productivity, leading to a lack of precise constraints on these relationships. Here, we use analyses of stable carbon isotopes (δ13Corg, δ13Ccarb, and Δ13C), total organic carbon (TOC), detrital proxies (Al, Si, Ti, K), redox-sensitive (RSTE: U, V, Mo) and productive-sensitive (PSTE: P, Cu, Ni) trace elements from a continuous, predominantly carbonate succession of the Kazhdumi Intrashelf Basin to evaluate the culprits for the OAE1a-associated changes in bottom-water oxygenation, organic-rich layer formation, and biotic shifts along the Arabian margin of the Neo-Tethys. Concentrations of Al-normalized RSTE and TOC values indicate that the bottom water conditions ranged from oxic prior to and at the onset of the OAE 1a (carbon-isotope segments C2 to basalmost C4 sensu Menegatti et al., 1998), to anoxic-suboxic but not euxinic (Mo < 25 ppm) during the lower C4 through C5 + C6 segments, and then returned to oxic-suboxic in the remaining C5 + C6 segment. The increase in Al-normalized PSTE coupled with TOC concentrations in the basal C4 is coeval with a change from predominantly orbitolinid-ostreid to planktic foraminifera-radiolarian biota. The periodically high productivity, driven both by the surface-water productivity as well as by phosphorus recycling from the sediments, continued through the C5 + C6 segments as evidenced by matching Si/Al and PSTE peaks (Cu/Al and Ni/Al). The study sheds new light on the causes of variations in bottom-water deoxygenation, organic content, nutrient availability, and biotic shifts in semi-restricted, relatively deep (>100 m), continental-margin basins during major oceanic perturbations.

A Tidal Flat Adjacent to a Fringe Mangrove Forest Mitigates pCO2 Increases and Enhances Lateral Export of Dissolved Carbon

Carbon export from mangrove forests to the oceans partly acts as a sink for atmospheric CO2, exceeding the rate of carbon burial in mangrove soils. Primary production in ecosystems adjacent to mangroves may prevent degassing and enhance further carbon export from mangroves to the oceans. In this study, we continuously monitored carbonate chemistry parameters (pCO2, dissolved inorganic carbon (DIC), total alkalinity (TA)) and dissolved organic carbon (DOC) in a tidal flat adjacent to a fringe mangrove forest over a spring-neap tidal cycle. Mean pCO2 during the entire period was 923 ± 318 μatm, and the export of TA, DIC, and DOC from the mangroves to the ocean was 36 ± 26 mmol m−2 d−1, 42 ± 39 mmol m−2 d−1, and 10 ± 9 mmol m−2 d−1, respectively. Semi-monthly pCO2 variations in the mangrove front were controlled by the tidal level during spring tide and by photosynthesis and respiration on the tidal flat during neap tide. This means that during neap tide, photosynthesis on the tidal flat offset the increase in pCO2 caused by the porewater export from the mangrove soil. The DIC/TA export ratio in this study was 1.17 ± 0.08, which was lower than the global average of 1.41 ± 1.39, indicating that the tidal flat adjacent to the mangrove forest may act as a buffer zone to mitigate the increase in pCO2, resulting in much of the exported DIC being stored in the ocean.

Urbanization influences dissolved organic matter characteristics but microbes affect greenhouse gas concentrations in lakes

Recognition and prediction of dissolved organic matter (DOM) properties and greenhouse gas (GHG) emissions is critical to understanding climate change and the fate of carbon in aquatic ecosystems, but related data is challenging to interpret due to covariance in multiple natural and anthropogenic variables with high spatial and temporal heterogeneity. Here, machine learning modeling combined with environmental analysis reveals that urbanization (e.g., population density and artificial surfaces) rather than geography determines DOM composition and properties in lakes. The structure of the bacterial community is the dominant factor determining GHG emissions from lakes. Urbanization increases DOM bioavailability and decreases the DOM degradation index (Ideg), increasing the potential for DOM conversion into inorganic carbon in lakes. The traditional fossil fuel-based path (SSP5) scenario increases carbon emission potential. Land conversion from water bodies into artificial surfaces causes organic carbon burial. It is predicted that increased urbanization will accelerate the carbon cycle in lake ecosystems in the future, which deserves attention in climate models and in the management of global warming.

The upper Cambrian SPICE carbon isotope excursion from the Alborz Ranges, northeastern Iran

Seawater chemistry has been highly variable during Earth's history. Whereas variations in the ratio of organic relative to inorganic carbon burial in oceanic basins are considered to trigger positive shifts in global seawater δ13C values, those in δ18O are related to Earth's climate fluctuations. Positive shifts in the carbon isotope record known from Cambrian marine carbonates on a global scale have provided invaluable stratigraphic insights. Carbon and oxygen isotope data from Middle-Upper Cambrian shallow marine carbonates of the Mila Formation, eastern Alborz Ranges, Iran are presented herein. Accordingly, the Furongian SPICE (Steptoean Positive Carbon Isotope Excursion) event is documented from the Qol-Qol Member of the Mila Formation at two stratigraphic exposures where this prominent and globally recorded isotopic event is detected in micrites/skeletal limestone with a maximum δ13C value of +4‰ at both localities. The positive shift initiates at the base, reaches the climax in the middle, and then declines in upper part of the Qol-Qol Member. Therefore, the base of the latter denotes the base of Furongian Series/Paibian Stage in the Alborz Ranges. The event occurred in the Palaeadotes–Torifera and Prochuangia trilobite Zones but detailed biostratigraphic studies are yet to be undertaken. The occurrence of glauconite and abundant benthic fauna in some intervals within the rock unit can be interpreted as recurring slightly-reduced and oxygenated sea floor circumstances during deposition of the Qol-Qol Member of the Mila Formation.

Salinity, mineralogy, porosity, and hydrodynamics as drivers of carbon burial in urban mangroves from a megacity

Mangrove ecosystems are an important blue carbon store but exhibit considerable variation in soil carbon stocks globally. Unravelling the conditions controlling carbon stock is critical for assessing current and future carbon budgets. Mangrove soil biogeochemical cycles can strongly influence carbon storage capacities. We thus investigated carbon sequestration and the environmental parameters shaping variability in biogeochemical cycling and carbon storage in sediment samples from four mangrove sites along an estuarine-to-marine gradient in Hong Kong, a megacity. Our results showed that organic matter in Hong Kong mangroves is sourced principally from autochthonous mangrove plants. Total nitrogen was higher in the freshwater-influenced sites and supplied from different sources. Marine-influenced sites had larger sulfur fractionations, reflecting higher marine-sourced sulfate concentrations and indicating a relatively open sulfate system. We estimated an average organic carbon stock of 115 ± 8 Mg C ha−1 in the upper 100 cm soil layer placing Hong Kong mangroves at the lower end of the global spectrum of the soil carbon stock. Carbon accumulation was found to be driven by a combination of higher total organic matter inputs, soil fluxes, and porosity. Notably, despite having the highest mass-specific soil organic carbon contents, Mai Po had the lowest integrated soil organic carbon storage (77 ± 3 Mg C ha−1). This was primarily due to lower sediment density and higher tidal pumping leading to a decrease in carbon retention. Total organic matter input, sediment characteristics, and hydrodynamics were the main factors influencing soil organic carbon storage. Overall, our results suggest that (1) while multiple parameters can enhance soil organic carbon content and increase carbon storage capacities, (2) hydrodynamics and sediment characteristics can increase the potential for leakage of carbon, and (3) high carbon content does not always equal high carbon sequestration and stock.

Erosional modulation of the balance between alkalinity and acid generation from rock weathering

The balance between alkalinity generation by carbonate and silicate weathering and sulfuric acid generation by sulfide weathering controls the effect of weathering on atmospheric pCO2 over geologic timescales. How this balance varies across environmental gradients remains poorly constrained. Here, we analyze this balance across an erosional gradient of two orders of magnitude in the Three Rivers (Yangtze, Mekong, and Salween Rivers) Headwater Region (TRHR), eastern Qinghai-Tibet Plateau (QTP). By employing major element chemistry and multiple isotopes (δ34SSO4, δ18OSO4, and δ18OH2O) coupled with forward and inverse approaches, we unmix contributions of silicates, carbonates, evaporites, and sulfides to the total weathering budget. Across the TRHR, riverine SO42– is derived mainly from a mixture of an evaporite source with uniform values of δ34SSO4 and δ18OSO4, and a sulfide source that contributes highly variable values of δ34S (−12.2 ‰ to +4.1 ‰) and δ18O (−17.7 ‰ to −1.6 ‰). Contributions of sulfide oxidation to riverine SO42– vary from 16 % to 94 %, and sulfuric acid consumes 6 % to 63 % of the alkalinity produced by weathering. The fractions of weathering alkalinity derived from carbonate weathering range from 36 % to 98 % relative to silicate weathering. The combination of silicate, carbonate, and sulfide weathering suggests that the instantaneous weathering fluxes of most sampled catchments in the TRHR act as a sink of atmospheric CO2 over timescales shorter than marine carbonate burial (∼104 years), but as a carbon source over timescales longer than carbonate burial and shorter than sulfide burial (∼107 years). The spatial variability of the balance between alkalinity and acid generation, and, thus, the relationship between chemical weathering and atmospheric pCO2, are largely dependent on lithology. However, within comparable lithologic settings, sulfide and carbonate weathering rates rise with increasing erosion, whereas silicate weathering rates remain constant. Consequently, plateau weathering shifts from a sink to a source of atmospheric CO2 with increasing erosion. These findings suggest that sulfide weathering is more sensitive to erosion than carbonate and silicate weathering, and that it could play an important role in the long-term carbon cycle during mountain building.

Copper Catalysis-Based Amperometric Microsensors for Carbon Dioxide Monitoring.

A fast response microsensor that can detect the distribution of CO2 at the microscale level is essential for the observation of biophysiological activity, carbon flux, and carbon burial. Inspired by the previous success of Cu catalysis, we attempted to use this metal Cu material to develop an amperometric microsensor that can meet the requirements. Specifically, the ambient gases diffuse through a silicone membrane into a trap casing filled with an acidic CrCl2 solution, where the otherwise interfering O2 interferent is removed by a redox with Cr2+. The gases then diffuse through a second silicone membrane into an electrolyte, where CO2 is selectively reduced to methanol (CH3OH) at a Cu cathode through a carbon monoxide (CO) pathway. Due to the use of Cu catalysis at the WE tip, CO2 can be reduced at a less negative polarization (-470 mV) instead of the previously reported -1200 mV, thus avoiding hydrogen-evolution interference due to water from the byproduct or from water diffusion through the silicone membrane. This moderate polarization results in a stable baseline, making the microsensor suitable for long-term monitoring. Interferences from other gases, such as N2O, which may be of much concern in environmental monitoring, can be ignored. Applications and limitations are also discussed with a view to further improvement in the future.

Fossil‐Bearing Concretions of the Araripe Basin Accumulated During Oceanic Anoxic Event 1b

AbstractFossils from the Araripe Basin (northeastern Brazil) are known for their remarkable preservation of vertebrates and invertebrates, even including soft tissues. They occur in carbonate concretions within organic carbon‐rich strata assigned to the Romualdo Formation. Here we present integrated stable isotope, elemental and microfossil records from the Sítio Sobradinho outcrop, Araripe Basin, northeastern Brazil. Our results imply that black shales hosting fossil‐bearing carbonate concretions within the lower Romualdo Formation were deposited during Oceanic Anoxic Event (OAE) 1b (Kilian sub‐event). Our high‐resolution multi‐proxy approach allows identifying four phases of environmental evolution. After a pre‐event phase, an early phase (onset of the negative carbon isotope excursion—nCIE) of water column stratification and reduced oxygenation likely preconditioned the system for organic carbon burial and preservation. A second phase (peak nCIE) was characterized by an intensified hydrological cycle and continental runoff, as well as increased influx of terrestrial organic matter. High input of continent‐derived nutrients might have enhanced biological productivity in the epicontinental sea, ultimately leading to increased organic carbon fluxes and burial, as well as carbonate dissolution at the seafloor. All together, these paleoenvironmental conditions resulted in expansion of an oxygen minimum zone (OMZ), favoring taphonomic processes that led to the excellent preservation of diverse macro‐ and microfossils. The nCIE recovery phase was characterized by reduced nutrient supply and organic carbon burial. Organic carbon sequestration in such paleoenvironments likely contributed to the recovery (increase) of stable carbon isotope (δ13C) records in the deep ocean during the Kilian sub‐event of OAE 1b.

Long-term carbon sequestration in the Eocene of the Levant Basin through transport of organic carbon from nearshore to deep marine environments

This study addresses a specific component associated with mass transport complexes in marine systems: the role of hyperpycnal flows, dense shelf water cascading, submarine canyons, distributary channels, and other transport mechanisms in transferring organic matter from continental and shallow marine settings into deep-marine environments. We speculate that during the Eocene, allowing for only 0.1‰ of shelf carbon to be preserved through transport mechanisms would account for up to 13.7% of all organic carbon burial. As such, the potential to mobilize through this mechanism large quantities of organic carbon is significant.Our case study focuses on a 150 m Eocene sequence composed of organic-rich chalks interleaved with displaced neritic limestones. TOC values range between 1.5 and 14%, averaging 4.5%. Displaced limestones are composed of a variety of poorly cemented mud- and wackestones, with low-diversity assemblages of large benthic foraminifera associated with planktonic foraminifera, suggesting deposition under low-energy conditions within the oligophotic zone on the outer ramp. Transport overprints include soft-sediment deformation, partially lithified rip-ups, folds, small diapirs, bed-scale imbrication, brecciation and syn-sedimentary shear. These features indicate detachment, movement and emplacement following initial sedimentation, in some cases more than once. Emplacement occurs into a chalk facies that can vary in appearance from darker (higher TOC) and lighter (lower TOC) lithofacies.Combination between the sedimentological, petrophysical, and elemental analyses indicates shifts between autochthonous and allochthonous sedimentation, whereas the organic geochemical analysis reveals a correlation between modes of sedimentation and preservation/composition of organic matter. Organic richness seems to increase within intervals of allochthonous sedimentation, with lower TOC values within intervals of autochthonous sedimentation. Organic matter preservation is enhanced due to poor oxygenation of the sea floor, further depleted by rapid burial beneath mass-transport deposits, increasing sedimentation rates and thus organic matter preservation. Horizons rich in organic matter may be derived from three different sources: organic matter with a fingerprint of terrestrial sources (e.g., enhanced contribution of plant leaf waxes) transported from nearshore environments; an allochthonous marine fingerprint with sulfurized hopanoids, which seem to be reworked from pre-existing Cretaceous organic-rich carbonates entrained within fined-grained micro-turbidites in the para-autochthonous facies; and productivity-derived organic matter deposited on the seafloor of the deep marine environment.This study demonstrates how transport mechanisms allow for the long-term burial of organic carbon in marine systems. When taking into consideration similar processes reported to occur in the world oceans today, it is clear that sediment transport and long-term burial of organic carbon is a fundamental part of the global carbon cycle.

Sea-level changes control coastal organic carbon burial in the southern East China Sea during the late MIS 3

Sea-level changes control coastal organic carbon burial in the southern East China Sea during the late MIS 3

How to quantify blue carbon sequestration rates in seagrass meadow sediment: geochemical method and troubleshooting

Seagrasses take up carbon dioxide and transform it into organic carbon, some of which is buried in meadow sediments. Very high carbon burial rates have been claimed for seagrass meadows globally, and international protocols have been developed with a view to awarding carbon offset credits. However, recent geochemical work has shown that a misunderstanding of how marine sediment buries and processes organic carbon has led to overestimates of at least an order of magnitude. Common blue carbon methodology does not adequately account for bioturbation or remineralization in surface sediment, and there is often a conflation of standing stock with ongoing burial. To determine accurate seagrass carbon burial rates requires the following steps: (1) Determine the sediment accumulation rate below the surface mixed layer, using 210Pb and porosity; (2) Determine the burial concentration of organic carbon; (3) Multiply the sediment accumulation rate by the buried % organic carbon; (4) If excluding allochthonous carbon burial, use biomarkers to determine the proportion of seagrass-derived organic carbon; and (5) Account for the offset due to the burial of carbonate formed inside the meadow. Seagrass meadows provide valuable habitat and protect coastlines from erosion. They can also play a role in short-term carbon sequestration. However, if carbon credits are awarded based on inflated estimates and used to offset emissions elsewhere, the net effect could be an increase in carbon dioxide emissions to the atmosphere.

THE NUMERICAL ANALYSIS OF MIXING DEPTH AND THE THICKNESS OF BBL CONSIDERING THE SUBMERGED AQUATIC VEGETATION AND WIND STRESS

Mangrove forests, salt marshes, and seagrass meadows have a vital role in mitigating global warming (Duarte et al.,2013). They contribute about 50 percent of carbon burial in ocean sediments (Duarte et al., 2013). On the other hand, submerged aquatic vegetations (SAVs) significantly affect the mixing depth (the thickness of the upper layer), resulting in a change in carbon flux with the atmosphere. According to the numerical simulations in the previous studies, mixing depth becomes larger with increasing SAV's density and decreasing SAV’s height (Herb et al., 2005, Vilas et al., 2017). On the other hand, Coates et al. (2009) indicated that wind stress was not crucial for mixing depth in SAV’s meadow since it did not change the vertical turbulent kinetic energy (TKE) profile, except in the thin surface layer, thus mixing depth became similar even though wind speed varied under the effect of SAVs (Coates et al., 2009). In addition, it is crucial to estimate the thickness of the benthic boundary layer (BBL) because dissolved inorganic carbon elucidates from bottom sediments, affecting carbon flux. Therefore, we aim to clarify how wind stress impacts the mixing depth and thickness of BBL with SAVs.